Extensible ladder

ABSTRACT

An extensible ladder which is provided with at least two pairs of telescopically slidable stringers and in which each rung of the outer pair of stringers is divided longitudinally into two rung parts between which the rungs of the other pair of stringers may be shifted longitudinally or locked so as to be in alignment with the rungs of the inner stringers. This ladder may also be designed as a convertible ladder consisting of two of these extension ladders which are pivotably connected at one end to each other so as either to permit them to be pivoted to an acute angle relative to each other to form a self-supporting double ladder or to be pivoted so as to be in straight alignment with each other to form a long straight ladder.

United States Patent Kiimmerlin et al.

[54] EXTENSIBLE LADDER [72] inventors: Walter Kiimrnerlln, Gerokstrasse 6, 7120 Bissingen; Nikolaus Adalbert Kiirnmerlln, Wannenstrasse 53, 7220 Schwenningen, both of Germany [22] Filed: Jan.6, 1971 [21] Appl.No.: 104,395

[30] Foreign Application Priority Data Jan. 14,1970 Germany..........P 20 01 416.8

[52] US. Cl. .....................l82/24, 182/167, 182/207 [51] Int. Cl ..E06c 1/383 [58] Field of Search ..l82/207, 208, 209, 24, 195,

[56] Relerences Cited UNITED STATES PATENTS 2,880,920 4/]959 Glessner 182/208 501,406 7/1893 Campbell ..l82/l67 [451 Sept. 19, 1972 796,157 8/1953 Skeel 182/209 2,647,676 8/ l 953 Napiecinski 182/25 1,936,508 1 1/1933 Hanly 182/24 Primary Examiner-Reinaldo P. Mach-ado Attorney-Arthur O. Klein [57] ABSTRACT An extensible ladder which is provided with at least two pairs of telescopically slidable stringers and in which each rung of the outer pair of stringers is divided longitudinally into two rung parts between which the rungs of the other pair of stringers may be shifted longitudinally or locked so as to be in alignment with the rungs of the inner stringers. This ladder may also be designed as a convertible ladder consisting of two of these extension ladders which are pivotably connected at one end to each other so as either to permit them to be pivoted to an acute angle relative to each other to form a self-supporting double ladder or to be pivoted so as to be in straight alignrnent with each other to form a long straight ladder.

13 Claims, 10 Drawing Figures EXTENSIBLE LADDER The present invention relates to an extensible ladder which may be adjusted to different lengths and may also be employed for scaffolds and comprises at least two pairs of stringers each of which forms one section of the ladder which may be shifted in its longitudinal direction relative to the other section and may be locked thereto in different extended positions.

In most of the known extensible ladders of this type the stringers of the different ladder sections which may be shifted relative to each other are disposed laterally adjacent to and partly above each other and the stringers of one section are provided with .guide means which embrace the stringers of the other section. The thickness of such a ladder when not extended is therefore dependent upon the number of stringers on each side of the ladder which are disposed partly above each other. There is another known type of extensible ladder in which the stringers of one section are k guided between the stringers of the other section, for example, in grooves in the latter. Such a ladder when not extended also has a considerable thickness since the rungs of the two ladder sections are then disposed within two different planes above or in front of each other in the same manner as in a ladder of the firstmentioned type.

It is an object of the present invention to provide an extensible ladder which comprises at least two ladder sections which are adjustable relative to each other and which in its basic non-extended position requires a minimum of space but possesses a very high stability in this basic position and also in any extended position, and which may be very easily manipulated. For attaining this object, the invention provides that the rungs of the first ladder section are divided so that each of these rungs consists of two substantially parallel longitudinal members which are sufficiently spaced from each other so as to permit the rungs of the second ladder section to pass freely between them during an adjustment of the ladder or to be locked in alignment with the rungs of the second section.

Due to this construction, the ladder according to the invention requires in the non-extended position an amount of space which is equal to or only slightly larger than that which the first ladder section alone requires since the two members of each divided rung may have a relatively small width without impairing the safety of the person standing on such a rung.

One feature of some of the preferred embodiments of the invention which renders the entire ladder as well as its individual sections very stable and distortion-resistant and reduces the friction between the ladder sections when being shifted longitudinally relative to each other consists in providing each of the outer stringers of the first ladder section in the form of a pair of bars of substantially U-shaped cross section the open sides of which, however, face in opposite directions, and in securing the free ends of the outer rungs of the first ladder section to the outer sides of the substantially parallel arms of the outer stringers.

By making the stringers of such cross-sectional shapes, only a relatively small friction will occur between the two ladder sections when being shifted longitudinally relative to each other since only the lateral arms of the stringers will then slide along each other and may cause some friction, while by making the inner rungs of a suitable length, a sufficient clearance may be attained between the outer edges of the arms of the inner stringers and the crossbars of the outer stringers so that a frictional engagement between these edges and crossbars will be practically avoided. By making the stringers of such cross-sectional shapes the further advantage may be attained that the lower parts of the outer stringers may diverge toward their lower ends and thereby increase the steadiness of the ladder. Since according to the above-mentioned feature of the invention the outer rungs are secured to the outer sides of the parallel arms of the outer stringers, the construction of the ladder will be very simple and the costs of its production will be relatively low, and especially its structure will be extremely distortion-resistant.

According to one preferred embodiment of the invention, the outer rungs of the ladder are provided in the form of cylindrical tubes the ends of which are flattened for being secured to the outer stringers. By making the outer rungs of such a shape, not only the advantages are attained that the ladder has a high stability and a low weight, but also that the costs of its production are low and that it may be made of a very low thickness. This low thickness may be attained by flattening the ends of the tubes forming the outer rungs so as to be offset from the longitudinal axes of the main cylindrical parts of the tube to such an extent that these cylindrical parts will not extend between the inner stringers, provided the inner rungs are made of a suffciently low thickness or width so as not to engage with these inwardly projecting outer rung parts.

According to another preferred embodiment of the invention, the inner and outer rungs are provided in the form of tubular rods of a substantially parallelogramshaped cross section. The upper sides of these rungs are preferably provided with recesses in which non-skid inserts are embedded. The acute angles of the parallelograms are then made of a size in accordance with the normal angle at which the ladder is to be mounted for being used.

According to a further embodiment of the invention, the ladder is designed so as to permit it to be used either as a double or self-supporting ladder or as a straight ladder of a great length. This convertible ladder comprises two parts each of which may again be extensible and which are pivotably connected to each other at their upper ends and provided with releasable locking means for maintaining the two ladder parts at a certain acute angle to each other when the ladder is to be used as a double or self-supporting ladder. When these locking means are released or disengaged, the two ladder parts may be pivoted relative to each other so as to extend in the same direction and thus form a single or straight ladder of a great length.

Especially in connection with those embodiments of the invention in which the ladder is always to be mounted for use substantially at the same acute angle to a vertical plane, at least the upper surfaces of the rungs preferably form flat substantially horizontal tread surfaces of steps. This applies especially to double or self-supporting ladders and therefore also to the convertible ladder according to the embodiment of the invention as last described. Since when this ladder is converted from a double ladder to a single or straight ladder by swinging one ladder part about the other by a pivotal connection between the upper ends of the two ladder parts so that the second, upper ladder part will be in straight alignment with the first lower ladder part, the formerly horizontal tread surfaces of the steps of this second ladder part would now face downwardly and be inclined at an angle. Such inclined steps of this upper ladder part would, of course, be dangerous. In order to eliminate this disadvantages the invention further provides that at least one part of the double ladder, that is, the part which is to form the upper part of the ladder when converted into a straight ladder, be provided with upper and lower surfaces which are disposed at an angle to each other so that the actual tread surfaces of all of the steps of this ladder part will always be substantially horizontal regardless of whether the ladder is used as a double ladder or as a long single ladder. It is for this purpose advisable to provide each inner rung and the parts of each outer rung which are located at the outer or mounting side of one or both parts of the ladder, but at least of the ladder part which is to form the upper part when used as a straight ladder, in the form of tubular rods or bars of a trapezoidal cross section the upper surfaces of which are disposed within one common plane and the lower surface of which are disposed within another common plane. When this ladder is then used either as a double ladder or as a long straight ladder which is mounted at the proper angle to a vertical plane, all of the actual tread surfaces of the rungs or steps will be disposed parallel to each other and substantially within horizontal planes.

These and additional features and advantages of the present invention will become further apparent from the following detailed description thereof which is to be read with reference to the accompanying drawings, in which FIG. 1 shows a perspective view of a part of the length of one side of an extensible ladder according to the invention with two stringers which are telescopically engaging with and slidable along each other;

FIG. 2 shows a top view of the two stringers according to FIG. 1;

FIG. 3 shows a cross section of the rungs of one part of a double extensible ladder according to a second embodiment of the invention;

FIG. 4 shows a side view of a ladder according to a third embodiment of the invention in the extended position;

FIG. 5 shows a side view of the ladder according to FIG. 4 in the contracted position;

FIG. 6 shows, on a larger scale, a centrally interrupted cross section which is taken along the line VI- VI of FIG. 4;

FIG. 7 shows a cross section similar to that as shown in FIG. 6 of one side of an extensible ladder according to a fourth embodiment of the invention;

FIG. 8 shows a broken-off cross section which is taken along the line VIII-VIII of FIG. 7;

FIG. 9 shows a side view of a part of a double ladder according to a fifth embodiment of the invention; while FIG. 10 shows a side view of a part of the ladder as shown in FIG. 9 when used as a ladder which is to be supported on a wall or other object.

In the drawings, FIGS. 1 and 2 illustrate an extensible ladder according to a first embodiment of the invention in which each of the rungs in divided into two longitudinal parts. This ladder comprises a first ladder section 302 which has two equal outer stringers 304, only one being shown, which are of the same shape and dimensions and each of which consists of a tube of a rectangular cross section the inner side wall of which is provided with a wide longitudinal slot. Each of these outer stringers 304 embraces and is separated by a small clearance from an inner stringer 313 of a second ladder section 314 which is slidable in its longitudinal direction within and along the outer stringer 304 and has a substantially corresponding cross section.

The two parts of the inner side wall of each outer stringer 304 adjacent to the longitudinal slot carry equal pairs of bars 305 and 305' of a U-shaped cross section which form the divided rungs of the first ladder section. The ends of the two bars of each pair are rigidly secured at equal levels and equal distances from each other to the outer stringers 304 and so that their open sides face each other, and the adjacent pairs of these bars 305 and 305' are likewise spaced at equal distances from each other. Since the two bars of each pair of bars 305 and 305' are spaced at a relatively large distance from each other, at least the upper arms of those bars on which the user of the ladder may stand may be made of a short length.

The rungs of the second ladder section 314 likewise consist of equal pairs of rails of a U-shaped cross section 323 and 323' and the rails of each pair are likewise spaced from each other, but only by a narrow gap. Whereas the rungs 305, 305' of the first ladder section 302 are secured to the slotted walls of the outer stringer 304, the rungs 323 and 323 are secured to the inner sides of the opposite unslotted walls of the inner stringers 313 and extended between and are spaced from the edges of the longitudinal slots in the outer stringers 304 so as to be movable along these slots. By dividing each of the rungs of the two ladder sections so as to form two U-shaped rails the open sides of which face each other, the entire ladder may be made of a very light weight, even through it has a very high stability.

For locking the outer and inner stringers 304 and 313 in any desired position to each other to which they have been adjusted longitudinally relative to each other, the outer stringers are provided with locking devices which comprise locking pins 306 and 307 which are slidable in their axial directions against the action of return springs andare adapted to be inserted into bores in the side walls of the inner stringers 313. The ladder as shown in FIGS. 1 and 2 is thus adjustable in steps, each step being equal to the distance between two adjacent pairs of outer and inner rung parts or equal to one half of this distance, if so desired.

If the ladder according to FIGS. 1 and 2 should form a double or self-supporting ladder or should always be inclined substantially at a certain acute angle to a vertical plane, the rungs may be modified as indicated in FIG. 3 so that their upper surfaces which are formed by the upper arms of the U-shaped rails 405, 405' and 423 are disposed within horizontal planes when the ladder is in its operative position.

According to the third embodiment of the invention, as illustrated in FIGS. 4 to 6, the outer stringers 704 of the first ladder section 702 consist of opposite rails of a U-shaped cross section the open sides of which face each other and both of which are bent outwardly at the same points so that the lower parts 704' diverge out wardly from each other toward their ends so as to improve the stability and steadiness of the ladder. The inner stringers 713 of the second ladder section 714 which extend between the arms of the outer stringers 704 and are guided by the latter are likewise of a U- shaped cross section but their open sides face in opposite directions. The outer surfaces of the parallel arms of the inner stringers 713 are spaced from each other at a slightly smaller distance than the inner surfaces of the parallel arms of the outer stringers 704 so as to permit the second ladder section 714 to be easily pushed or pulled into and out of the first ladder section 702. The edges 740 and 704' of the parallel arms of each inner stringer 713 are bent inwardly toward each other and, by making the rungs of the second ladder section 714 of the proper length, these edge portions 740 and 704' are separated by small clearances from the transverse walls of the outer stringers 704 so as to be easily movable along these walls. The parts 705 and 705' of the outer rungs of the first ladder section 702 are mainly of a cylindrical tubular shape and are flattened toward their ends which are to be secured to the outer sides of the parallel arms of the outer stringers 704. These parts 705 and 705' may be easily produced by placing cylindrical tubes of the proper length upon a solid base and by then compressing their ends by pressing them flat upon this base. The flat ends of the tubes will thus extend very eccentrically to the axes of the main cylindrical parts of these rungs, so that, when these ends are welded upon the outer surfaces of the arms of the outer stringers 704, the main cylindrical parts of the outer rungs 705 and 705' will project between the inner stringers 713. The inner rungs 723 which connect the inner stringers 713 to each other are made of a relatively small width so as not to engage with the outer rungs 705 and 705'.

FIGS. 7 and 8 illustrate an extensible ladder according to a fourth embodiment of the invention. This ladder may be employed in those cases in which, regardless of the total length to which it may be extended or contracted, the ladder will always be mounted at substantially the same acute angle to a vertical plane. This ladder differs primarily from that as illustrated in FIGS. 4 to 6 by making the inner stringers 813 of a solid construction and of a substantially U-shaped cross section by providing each of them with a dovetailed recess 841 which extends in the longitudinal direction of the stringer and the open side of which faces the inner wall of the associated outer stringer 804 and by making the inner and outer rungs 823 and 805, 805' of a substantially parallelogram-shaped cross section in which the upper sides of the rungs are provided with recesses 842 in which non-skid inserts 843 are embedded which form wide tread surfaces which extend horizontally when the ladder is mounted at the proper angle to a vertical plane. The parts 805 and 805 of the outer rungs may be secured to the outer stringers 804 in the same manner as the corresponding parts are secured in the ladder according to FIGS. 4 to 6 and the inner stringers 814 are also made of such dimensions relative to those of the outer stringers 804 as to be easily slidable in their longitudinal directions within the outer stringers.

FIGS. 9 and 10 finally illustrate a fifth embodiment of the invention in the form of an extensible ladder which may be employed either as a double or self-supporting ladder or as a straight ladder of a considerable length. This ladder comprises two arms 944 and 945 which may be of a construction similar to that of the ladder according to FIG. 3. The ladder sections 902 of the two arms 944 and 945 are connected to each other at their upper ends so as to permit the ladder arm 954 to be pivoted about the other arm 944, and they are additionally connected to each other by a releasable locking device 946. When this locking device 946 is released or disengaged, the two ladder arms 944 and 945 may thus be pivoted about an angle of I relative to each other so as to form a long straight ladder as shown in FIG. 10 or, when the parts of the locking device 946 are connected to each other, the two ladder arms may be pivoted relative to each other so as to extend at an acute angle to each other and thus to form a double or self-supporting ladder as shown in FIG. 9.

For the ladder as shown in FIGS. 9 and 10 it is of importance that the rungs are made of a proper construction. They should be designed so as to permit them to be stepped on from one side or the other depending upon whether the ladder is used as a double ladder or as a straight ladder. All inner rungs 923 and all parts 905 of the outer rungs 905, 905, 947 are for this purpose made of tubular bars of such a trapezoidal cross section and secured in such a manner that all upper and lower surfaces 948 and 949 of these rungs will be disposed within corresponding angles relative to the associated stringers. Therefore, if after the locking device has been disengaged, the two ladder arms 944 and 945 are pivoted about their upper ends so as to be in straight alignment with each other, and if the long straight ladder which is thus formed is mounted and supported at the proper acute angle, that is, at the same angle at which each of the ladder arms was inclined when the ladder was used as a double or self-supporting ladder as shown in FIG. 9, those of the rung or step surfaces 948 and 949 of both ladder arms 944 and 945 which then face upwardly will all be disposed within substantially horizontal planes regardless of whether the ladder arm 944 is pivoted about the upper end of the other ladder arm 945 or vice versa. This, of course, presupposes that the slidable section of each ladder arm 944 and 945 is adapted to be locked to the associated ladder section in such positions that the trapezoidal parts 905 of the outer rungs 905, 905', 947 which are located at the outer side or sides of the ladder. that is, at the side from which a person mounts the ladder or ladder arm, and the associated trapezoidal inner rungs 923 together also form trapezoids, as seen in cross section.

The two parts 905' and 947 of each outer rung at the inner side or sides of the ladder may also be combined with each other so as to form a single tubular bar of a trapezoidal cross section the parallel sides of which extend parallel to the longitudinal axes of the associated stringers, while the two other sides are disposed within the two different planes of the surfaces 948 and 949.

Although our invention has been illustrated and described with reference to the preferred embodiments thereof, we wish to have it understood that it is in no way limited to the details of such embodiments but is capable of numerous modifications within the scope of the appended claims.

Having thus fully disclosed our invention, what we claim is:

1. An extensible ladder comprising at least two pairs of stringers and a plurality of rungs extending transverse to and connecting each pair of stringers, one of said pairs including its rungs forming a first ladder section and the other pair including its rungs forming a second ladder section, said stringers having a cross-sectional shape so that said stringers of said first section form outer stringers which partly encompass the inner stringers of said second section so that said inner stringers are telescopically slidable within and guided by said outer stringers to permit said second section to be shifted in its longitudinal direction relative to said first section, each of said rungs of said first section being divided in its longitudinal direction to form two outer rung parts which are spaced at such an inner clearance distance from each other to permit the inner rungs of said second section to pass between them, and to be freely slidable in said longitudinal direction relative to said outer rung parts, and said outer rung parts having a maximum transverse dimension relative to the longitudinal axis of said inner stringers which is less than double the width of said inner stringers, and further comprising means for locking said two sections to each other in different positions of adjustment of said second section relative to said first section.

2. An extensible ladder as defined in claim 1, in which each of said rung parts of said first section forms a bar of a substantially U-shaped cross section having an open side facing in a direction transverse to said longitudinal direction.

3. An extensible ladder as defined in claim 1, in which said outer stringers of said first section form bars of a substantially U-shaped cross section having open sides facing each other, said inner stringers of said second section also forming bars of a substantially U- shaped cross section having open sides facing in opposite directions to each other the ends of said outer rung parts of said first section being rigidly secured to the outer sides of the substantially parallel arms of said outer stringers.

4. An extensible ladder as defined in claim 3, in which the edge portions of the substantially parallel arms of each inner stringer of said second section adjacent to the open side thereof are bent inwardly toward each other.

5. An extensible ladder as defined in claim 3, in which for providing each inner stringer with a substantially U-shaped cross section, said inner stringer is provided with a dovetailed recess extending in its longitudinal direction.

6. An extensible ladder as defined in claim 3, in which said outer rung parts of said first section form cylindrical tubes the ends of which are pressed fiat and are secured to the outer sides of the substantially parallel arms of said outer stringer.

7. An extensible ladder as defined in claim 3, in which said inner and outer rungs form tubular bars having a substantially parallelogram-shaped cross section.

8. An extensible ladder as defined in claim 7, in which the upper sides of said rungs are provided with recesses, and further comprising non-skid inserts emi i $n fiiififislzfifier comprising two of said extensible ladders as defined in claim 3, further comprising means for pivotably connecting said two extensible ladders at their upper ends to each other, limiting means normally connecting said two extensible ladders to each other for determining the normal angle to which said two extensible ladders are pivotable relative to a vertical plane when said convertible ladder is to be used as a self-supporting ladder, said limiting means being adapted to be disengaged so as to permit a first of said two extensible ladders to be pivoted relative to the second of said two extensible ladders to a position in which both of said extensible ladders are in straight alignment with each other and together form a longer straight ladder.

10. A convertible ladder as defined in claim 9, in which each of said rungs on at least said first extensible ladder which is adapted to be pivoted about said second extensible ladder so as to form the upper part of said longer straight ladder has first and second surfaces extending at an acute angle to each other, said first surfaces extending within a common plane and forming horizontal tread surfaces when said convertible ladder is used as a self-supporting ladder, and said second surfaces extending within a common plane and forming horizontal tread surfaces when said two extensible ladders are in alignment with each other to form said longer straight ladder and when said longer straight ladder is mounted at said normal angle to a vertical plane.

11. A convertible ladder as defined in claim 10, in which all of the inner rungs at least of said first extensible ladder and all of the parts of the outer rungs of said first extensible ladder which are located at the outer side of said longer straight ladder at which said ladder is mounted by a user form tubular bars having a trapezoidal cross section.

12. A convertible ladder as defined in claim 11, in which the parts of each outer rung at least of said first extensible ladder which are located at the side of said longer straight ladder opposite to said outer side thereof form two tubular bars having a parallelogramshaped cross section.

13. A convertible ladder as defined in claim 11, in which the parts of said outer rungs at least of said first extensible ladder which are located at the side of said longer straight ladder opposite to said outer side thereof form tubular bars having a trapezoidal cross section.

l i i i 

1. An extensible ladder comprising at least two pairs of stringers and a plurality of rungs extending transverse to and connecting each pair of stringers, one of said pairs including its rungs forming a first ladder section and the other pair including its rungs forming a second ladder section, said stringers having a cross-sectional shape so that said stringers of said first section form outer stringers which partly encompass the inner stringers of said second section so that said inner stringers are telescopically slidable within and guided by said outer stringers to permit said second section to be shifted in its longitudinal direction relative to said first section, each of said rungs of said first section being divided in its longitudinal direction to form two outer rung parts which are spaced at such an inner clearance distance from each other to permit the inner rungs of said second section to pass between them, and to be freely slidable in said longitudinal direction relative to said outer rung parts, and said outer rung parts having a maximum transverse dimension relative to the longitudinal axis of said inner stringers which is less than double the width of said inner stringers, and further comprising means for locking said two sections to each other in different positions of adjustment of said second section relative to said first section.
 2. An extensible ladder as defined in claim 1, in which each of said rung parts of said first section forms a bar of a substantially U-shaped cross section having an open side facing in a direction transverse to said longitudinal direction.
 3. An extensible ladder as defined in claim 1, in which said outer stringers of said first section form bars of a substantially U-shaped cross section having open sides facing each other, said inner stringers of said second section also forMing bars of a substantially U-shaped cross section having open sides facing in opposite directions to each other, the ends of said outer rung parts of said first section being rigidly secured to the outer sides of the substantially parallel arms of said outer stringers.
 4. An extensible ladder as defined in claim 3, in which the edge portions of the substantially parallel arms of each inner stringer of said second section adjacent to the open side thereof are bent inwardly toward each other.
 5. An extensible ladder as defined in claim 3, in which for providing each inner stringer with a substantially U-shaped cross section, said inner stringer is provided with a dovetailed recess extending in its longitudinal direction.
 6. An extensible ladder as defined in claim 3, in which said outer rung parts of said first section form cylindrical tubes the ends of which are pressed flat and are secured to the outer sides of the substantially parallel arms of said outer stringer.
 7. An extensible ladder as defined in claim 3, in which said inner and outer rungs form tubular bars having a substantially parallelogram-shaped cross section.
 8. An extensible ladder as defined in claim 7, in which the upper sides of said rungs are provided with recesses, and further comprising non-skid inserts embedded in said recesses.
 9. A convertible ladder comprising two of said extensible ladders as defined in claim 3, further comprising means for pivotably connecting said two extensible ladders at their upper ends to each other, limiting means normally connecting said two extensible ladders to each other for determining the normal angle to which said two extensible ladders are pivotable relative to a vertical plane when said convertible ladder is to be used as a self-supporting ladder, said limiting means being adapted to be disengaged so as to permit a first of said two extensible ladders to be pivoted relative to the second of said two extensible ladders to a position in which both of said extensible ladders are in straight alignment with each other and together form a longer straight ladder.
 10. A convertible ladder as defined in claim 9, in which each of said rungs on at least said first extensible ladder which is adapted to be pivoted about said second extensible ladder so as to form the upper part of said longer straight ladder has first and second surfaces extending at an acute angle to each other, said first surfaces extending within a common plane and forming horizontal tread surfaces when said convertible ladder is used as a self-supporting ladder, and said second surfaces extending within a common plane and forming horizontal tread surfaces when said two extensible ladders are in alignment with each other to form said longer straight ladder and when said longer straight ladder is mounted at said normal angle to a vertical plane.
 11. A convertible ladder as defined in claim 10, in which all of the inner rungs at least of said first extensible ladder and all of the parts of the outer rungs of said first extensible ladder which are located at the outer side of said longer straight ladder at which said ladder is mounted by a user form tubular bars having a trapezoidal cross section.
 12. A convertible ladder as defined in claim 11, in which the parts of each outer rung at least of said first extensible ladder which are located at the side of said longer straight ladder opposite to said outer side thereof form two tubular bars having a parallelogram-shaped cross section.
 13. A convertible ladder as defined in claim 11, in which the parts of said outer rungs at least of said first extensible ladder which are located at the side of said longer straight ladder opposite to said outer side thereof form tubular bars having a trapezoidal cross section. 